Determination of Nucleon's Pseudoscalar Form Factor in MuCap Experiment

1. Determination of the nucleon’s pseudoscalar form factor in the MuCap experiment A.Vorobyov on behalf of MuCap collaboration HSQCD 2014 Gatchina 30.06.2014

2. Overview • Recent progress in studies of muon capture rates on proton, deuteron, and He3 made by the MuCap collaboration at PSI. • First determination of the nucleon’s pseudoscalar form factor. • Critical probe for Heavy Baryon Chiral Perturbation Theory. • Determination of Low Energy Constants for Effective Field Theories of two and three nucleon systems

3. MuCap collaboration Petersburg Nuclear Physics Institute Paul Scherrer Institute , SwitzerlandUniversity of Wasington, USA Regis University, USAUniversity of Kentucky, USABoston University, USA University of South Carolina, USA Université Catholique de Louvain, Belgium

4. Muon Capture on Proton -+ p  m+ n Chiral Effective Theories 4

5. Muon Capture on Proton -+ p  m+ n p n W µ ν 5

6. Muon Capture on Proton -+ p  m+ n p n W µ ν 6

7. Muon Capture on Proton - + p µ+ n p n qc2 = - 0.88 mµ2 W νµ µ gv(qc2) = 0.9755(5) values and q2 dependence known from gM(qc2) = 3.5821(25) EMform factors via CVC gA (qc2) =1.251(4) gA(0)=1.2701(25) from neutron β-decay, gP(expt) = ?q2 dependence from neutrino scattering ( ( µp-capture offers a unique way to determine gP(qc2)

8. gpNN n p p Fp m- nm Theoretical predictions for gP •Partial conservation of axial current (PCAC) • Heavy Baryon chiral perturbation theory (ChPT) W g P(qc2) = (8.74  0.23 ) – (0.48  0.02) = 8.26  0.23 PCAC pole term ChPT leading order one loop two-loop <1% Recent reviews:V. Bernard et al., Nucl. Part. Phys. 28 (2002), R1T. Gorringe, H. Fearing, Rev. Mod. Physics 76 (2004) 31

9. 45 years of Efforts to Determine gP OMC RMC - + p  n +  + g “ Radiative muon capture in hydrogen was carried out only recently with the result that the derived gP was almost 50% too high. If this result is correct, it would be a sign of new physics... ’’ — Lincoln Wolfenstein (Ann.Rev.Nucl.Part.Sci. 2003) 9

10. Pioneers of muon capture experiments 1969 Bologna-Pisa-CERN H2 –target 8 atm Emilio Zavattini 1927-2007 gP = 12 ± 5 1973 Dubna group H2 –target 41 atm gP = 9 ± 7

11. Status of µp-capture experiments Two experimental methods to measure ΛC Neutron detection Life time measurement CPT inv - + p  µ+ n Br=0.16% µ±p → e± νe νµ ΛC Nn/Nµ ΛC = 1/τµ+ - 1/τµ- *) First direct observation of µp-capture

12. ppμ ppμ LT = 12 s-1 triplet (F=1) pμ↑↑ Lortho=506s-1 Lpara=200s-1 λop μ ppμ ppμ f λppm ortho (J=1) para (J=0) pμ↑↓ singlet(F=0) LS= 710s-1 From which muonic state the muon capture occurs ? n+n

13. 1 % LH2 100% LH2 pm ppmO pm ppmO ppmP ppmP time (ms) Pµ - PPµ(ortho) - PPµ(para) population

14. Precise Theory vs. of Exp. Efforts gP - + p m+ n + g@TRIUMF 1996 ChPT - + p m+ n @ Saclay 1981 exp1 exp2 theory • lOP(ms-1) H2 –target 8 atm 1969 gP = 12 ± 5

15. Main requirements • The H2 gas pressure should not exceed 10 bar to provide dominant (µ-p)1S state. • The muon capture rate Λs in the reaction - + p  (µ-p)1S → µ+ n BR=0.16% should be measured to 1% precision • To reach such precision, one should measure the µ- life time in hydrogen with 10-5 precision. ΛS

16. Sensitivity of ΛS to form factors Contributes 0.45% uncertainty to measured S Theory predicts gP with ~ 3% precision Uncertainties in other form factors set the limit : =0.45% or =3% To approach this limit, one should measure ΛS with ~0.5% precision Examples: 10% 56% 1.0% 6.1% 0.5% 3.8%

17. High precision muon capture experiments based on New experimental method developed at PNPI Unique muon channel at PSI PSI meson factory MuCap setup

18. Strategy of MuCap experiment (µ-p)1S • H2 gas target at 10 atm

19. Strategy of MuCap experiment (µ-p)1S • H2 gas target at 10 atm • Lifetime method ΛS = 1/τµ+ - 1/τµ- 10 -5 precision both in µ- and µ+ life times → δΛS/ΛS = 1%

20. Strategy of MuCap experiment (µ-p)1S • H2 gas target at 10 atm • Lifetime method ΛS = 1/τµ+ - 1/τµ- 10 -5precision both in µ- and µ+ life times → δΛS/ΛS = 1% • >1010 muon decay events High data taking rate

21. Strategy of MuCap experiment (µ-p)1S • H2 gas target at 10 atm • Lifetime method ΛS = 1/τµ+ - 1/τµ- 10 -5precision both in µ- and µ+ life times → δΛS/ΛS = 1% •>1010 muon decayevents High data taking rate • Clean muon stop selection No wall effects

22. Strategy of MuCap experiment (µ-p)1S • H2 gas target at 10 atm • Lifetime method ΛS = 1/τµ+ - 1/τµ- 10 -5precision both in µ- and µ+ life times → δΛS/ΛS = 1% •>1010 muon decayevents High data taking rate • Clean muon stop selection No wall effects • Low background < 10-4

23. Strategy of MuCap experiment (µ-p)1S • H2 gas target at 10 atm • Lifetime method ΛS = 1/τµ+ - 1/τµ- 10-5 precision both in µ- and µ+ life times → δΛS/ΛS = 1% •>1010 muon decayevents High data taking rate • Clean muon stop selection No wall effects • Low background < 10-4 • Ultra clean protium impurities with Z>1 less than 10 ppb (1 ppb = 10-9) deuterium concentration less than 100 ppb

24. Time projection chamber, TPC H2 gas 10 atm room temperature - 40kV µ E ………………… . . . . . . . . . . . . G +5kV G Cathode strips Anode wires Sensitive volume 15 x 12 x 28 cm3 Muon stop selection inside the sensitive volume far enough from all chamber materials 3D detection of muon track X- cathode strips Y- drift time Z- anode wires Y y x x z z

25. eSC hodoscope ePC2 ePC1 e µSC 40kV µ E ………………… . . . . . . . . . . . . G 5kV µPC G Cathode strips Anode wires tµ = t eSC – t µSC e-µ space correlation reduces background

28. Cryogenic circulation-purification hydrogen gas system N2 less than 10 ppb O2 less than 10 ppb TPC provides control for impurities on a level of 10 ppb Gas purification system

29. Isotopic purity of protium Deuterium concentration in hydrogen gas Natural gas 140 ppm Best on market 2 ppm Produced at PNPI for MuCap ≤ 6 ppb Accelerator mass spectrometry at ETH in Zurich Reached sensitivity 6 ppb Cryogenic isotopic exchange column

30. Kicker Plates m detector m- TPC +12.5 kV -12.5 kV 50 ns switching time Data taking rate PSI muon channel can provide ~ 70 kHz muons MuCap could use only ~ 7kHz (to prevent pile up) New beam system (Muon-on-demand) constructed in 2005 allowed to increase the usable beam intensity up to 20 kHz ~3 times higher rate run2006 Muon-on demand system 10 000 hrs of beam time

31. e µ Reduction of background by µ-e vertex cut The impact cut can reduce the background to a level of 10-4-10-5

32. Determination of LS molecular formation bound state effect MuCap: precision measurement MuLan 32

33. Final result • λµ– = 455854.6 ± 5.4stat ± 5.1syst s-1 (MuCap) λµ+ = 455170.05 ± 0.46 s-1 (μLAN experiment), ΛSMuCap = 714.9 ± 5.4stat ± 5.1syst s-1 . gPMuCap(qc2) = 8.06 ±0.48±0.28 *) MuCap collaboration, Phys. Rev. Lett. 110, 022504 (2013). gPHBCPT = 8.26  0.23 V. Bernard, L. Elouadrhiri, and U.-G. Meissner, J. Phys.G28, R1 (2002). *) Based on updated calculations of Λs from A. Czarnecki, W.J. Marciano, A. Sirlin, Phys. Rev. Lett., 99, 032003 (2007)

34. The MuCap result does not depend on molecular OP-transitions gP(MuCap) = 8.06 ± 0.55 gP(theory) = 8.26 ± 0.23 34

35. Axial Vector gA PDG12 A. Garcia • PDG 2008gA(0)= 1.2695±0.0029 • PDG 2012gA(0)= 1.2701 ±0.0025 • Future ? gA(0)= 1.273 ? In this case gPMuCap = 8.24±0.55 gPTheory = 8.26±0.23 35

36. PHYSICS spotlighting exceptional research American Physical Society Synopsis: Sizing Up Quark Interactions Measurement of Muon Capture on the Proton to 1% Precision and Determination of the Pseudoscalar Coupling gP V. A. Andreev et al. (MuCap Collaboration) Phys. Rev. Lett. 110, 012504 (2013) Published January 3, 2013 Even though the radioactive decay of nuclei is mainly driven by the weak force, interactions between the quarks that make up the protons and neutrons in the nucleus can also affect the process. Calculating these effects with quantum chromodynamics (QCD), the theory describing the strong force interactions between quarks, is, however, mathematically cumbersome at the low energies associated with the nucleus. Instead, calculations are more tractable using an effective QCD theory, in which interactions are between bound quarks (mesons, protons and neutrons). Now, researchers running the muon capture (MuCap) experiment at the Paul Scherrer Institute in Switzerland have confirmed a long-standing prediction of the theory, known as chiral perturbation theory, boosting confidence that it can be used to accurately describe quark interactions in simple nuclei. Muon capture is like a beta-decay process run in reverse: a muon (a particle with the same charge as an electron, but 200 times the mass) interacts with a proton to produce a neutron and a neutrino. Among other factors, a dimensionless quantity called the “pseudoscalar coupling,” determines the rate of the reaction. Chiral perturbation theory says the coupling factor has a value of Gp , without a lot of wiggle room. But experimental data going back to the 1960s have shown the coupling could be anywhere between   and   . The MuCap collaboration, which measures the rate of the muon capture process by stopping a beam of muons in a low-density gas of pure hydrogen, has analyzed 30 terabytes of data to extract the pseudoscalar coupling with unprecedented precision. The value of their result, reported in Physical Review Letters, is 8.06+/-0.55 —in excellent agreement with the theoretical prediction. – Jessica Thomas

37. Muon capture rates on deuteron and He3

38. Quest for “unknown” Axial LEC LEC - low energy constants in Effective Field Theories 2-body system 1 LEC to be determined from µd capture experimental information scarce: ~100% uncertainty Measurement of µd capture rate to 1% precision will reduce uncertainty to ~15% 3-body system 2 LECs and additional complexity enter tritium beta decay Muon capture on He3 potential current 38

39. MECEFT L1A Next experiment “MuSun” m + d  n + n + n Mesurement of muon capture rate with 1% precision basic solar fusion reaction p + p  d + e+ +  key reactions for SNO  + d  p + p + e- (CC)  + d  p + n +  (NC) model-independent connection via EFT & L1A

40. Precise Experiment Needed 40

41. Muon Capture on He-3 µ- + 3He → 3H + νµ Et = 1.9 MeV This capture rate was measured in MuCap experiment with 0.3 % precision Λstat = 1496 ± 4 s-1 Physics Letters B417,224 (1998) The world precision was improved by a factor of 50 Since then, this result was a subject of various analyses with the goal to obtain the value of gp

42. m3He capture • + 3He → 3H + n • MuCap: 1496±4 /s (0.3%) • Pisa-JLab theory: 1494±21 /s gP(qc2)=8.2±0.7 42

43. Conclusions • The MuCap experiment is able to perform measurements of muon capture rates on proton and light nuclei on unprecedented level of precision. • This allowed for the first time to measure with high precision the nucleon’s pseudoscalar form factor, thus providing a critical probe of the Heavy Baryon Chyral Perturbation Theory. • The precision measurements of µd and µHe3 capture rates will allow to fix the Low Energy Constants in the Effective Field Theories of light nuclei.